Diffuse Optical Tomography (DOT) is a functional imaging modality that reveals physiological parameters, such as oxygen saturation of hemoglobin and blood flow, based on intrinsic tissue contrast, molecular tissue function, and gene-expression based on extrinsically administered fluorescent probes and beams. DOT offers several potential advantages over existing radiological techniques, such as non-invasive and non-ionizing imaging.
DOT imaging includes illuminating the tissue with a light source and measuring the light leaving the tissue with a sensor. A model of light propagation in the tissue is developed and parameterized in terms of the unknown scattering and/or absorption as a function of position in the tissue. Then, using the model together with the ensemble of images over all the sources, one attempts to invert the propagation model to recover the scatter and absorption parameters.
A DOT image is actually a quantified map of optical properties and can be used for quantitative three-dimensional imaging of intrinsic and extrinsic adsorption and scattering, as well as fluorophore lifetime and concentration in diffuse media such as tissue. These fundamental quantities can then be used to derive tissue oxy- and deoxy-hemoglobin concentrations, blood oxygen saturation, contract agent uptake, and organelle concentration. Such contrast mechanisms are important for practical applications such as the measurement of tissue metabolic activity, angiogenesis and permeability for cancer detection as well as characterizing molecular function.
A typical DOT system uses lasers so that specific chromophores are targeted and the forward model is calculated for the specific wavelengths used. Laser diodes have been customarily used as light sources since they produce adequate power and are stable and economical. Light is usually directed to and from tissue using fiber guides since this allows flexibility in the geometrical set-up used. For optical coupling, the fibers must be in contact with tissue or a matching fluid. Use of the matching fluids helps to eliminate reflections due to mismatches between indices of refraction between silica, air, and tissue.
Advanced DOT algorithms require good knowledge of the boundary geometry of the diffuse medium imaged in order to provide accurate forward models of light propagation within this medium. A forward model is a representation of the representative characteristics of the volume being studied. Currently, these boundary geometries are forced into simple, well known shapes such as cylinders, circles, or slabs. In addition to not accurately representing the shape of the specimen to be analyzed, the use of these shape forces the specimen to be analyzed to be physically coupled to the shape either directly or by the use of a matching fluid as discussed above.